1. Field of the Invention
The present invention relates to an acoustic surface wave device. More specifically, the present invention relates to an acoustic surface wave device having a non-symmetrical frequency characteristic with respect to the central frequency, for use in a video intermediate frequency circuit of a television receiver, for example.
2. Description of the Prior Art
A typical prior art acoustic surface wave device comprises a transducer including a piezoelectric material substrate of piezoelectric ceramic of such as PZT, a single crystal of such as LiNbO.sub.3 or a piezoelectric thin film of such as ZnO and a pair of interdigital electrodes are formed. Each of the interdigital electrodes comprises a comb shaped electrode, each comb shaped electrode comprising a plurality of electrode fingers and a common electrode for commonly connecting each of the electrode fingers at one end. The pair of comb shaped electrodes are disposed such that the plurality of electrode fingers of one comb shaped electrode are disposed in an interdigital manner with the plurality of electrode fingers of the other comb shaped electrode. One of the pair of the interdigital electrodes constitutes an input transducer and the other constitutes the output transducer. Because of the advantages that an acoustic surface wave device is small sized and requires no adjustment, an acoustic surface wave device has been utilized in various types of equipment. Of late, an acoustic surface wave device is also utilized as a filter in a video intermediate frequency circuit in television receivers. As is well known, a video intermediate frequency circuit of a television receiver must includes a sound trap. Thus, it is necessary to provide a filter having a frequency response that is non-symmetrical with respect to the central frequency, or the frequency f.sub.0 or .omega..sub.0 intermediate the picture signal frequency and the chroma signal frequency.
Conventionally, in order to achieve a non-symmetrical frequency response with respect to the central frequency in an acoustic surface wave device, a plurality of acoustic surface wave filters having different frequency responses have been used. However, utilization of a plurality of acoustic surface wave filters is not preferred because of an increased size of the apparatus. Of late, therefore, an attempt has been made to provide a single acoustic surface wave filter having a non-symmetrical frequency response. One approach is to change the distance between the centers of the two adjacent electrode fingers, i.e. the electrode pitch, in the propagating direction of the acoustic wave. Such type of acoustic surface wave filter may be referred to as a varying pitch type. More specifically, when a non-symmetrical frequency characteristic required for a video intermediate frequency filter, for example, is subjected to Fourier reverse conversion, then an impulse response as shown in FIG. 1 is obtained. The impulse response thus obtained includes an imaginary part as a result of Fourier reverse conversion. It is known that by forming the pattern of one of the input or output interdigital electrodes in association with the impulse response thus obtained, a desired frequency characteristic can be achieved in the acoustic surface wave device. More specifically, the electrode pitch is selected to be proportional to the period between the adjacent peak points in the impulse response as shown in FIG. 1 and the overlapping length of the adjacent electrode fingers, i.e. the acoustic surface wave exciting region, is selected to be proportional to the amplitude of the respective peak point in the impulse response as shown in FIG. 1. The acoustic surface wave filter thus obtained includes the interdigital electrodes of non-uniform electrode pitch, because of non-uniformity of the time period between the respective peak points where the imaginary part becomes zero, as seen from FIG. 1. and thus is of a so-called varying pitch type. Although such conventional approach achieves a desired characteristic in terms of the frequency response, non-uniformity of the electrode pitch and a possible extremely small electrode pitch for some electrode fingers make it difficult to design the electrode pattern and also makes it difficult to correctly form the electrode pattern through a photoetching process or the like and degrades the efficiency of manufacturing process.
In order to solve the above described disadvantage, therefore, different approaches have been proposed to achieve a non-symmetrical frequency characteristic using the interdigital electrodes of the equal pitch. One approach is an odd/even function method and the other is a mirror method.
The odd/even function method is described in detail, for example, in 1975 Ultrasocics Symposium Proceedings, IEEE Cat. #75 CHO 994-4SU "DESIGN CONSIDERATIONS FOR NONSYMMETRICAL SAW FILTERS". Now the odd/even function method will be briefly described. According to the odd/even function method, assuming the representation of a desired frequency characteristic to be H.sub.1 (.omega.), H.sub.2 (.omega.) then H.sub.1 (.omega.-.omega..sub.0)=H.sub.2 (.omega..sub.0 -.omega.). The relation between H.sub.1 (.omega.) and H.sub.2 (.omega.) is shown in FIG. 2. Assuming the even component, i.e. the symmetrical component to be H.sub.R (.omega.), the odd component, i.e. the non-symmetrical component to be H.sub.I (.omega.) and assuming further that H.sub.R (.omega.) and H.sub.I (.omega.) are defined by the following equations (1) and (2), then the relation of these is shown in FIG. 3. ##EQU1##
From the above described equations (1) and (2), H.sub.I (.omega.) is expressed by the following equation (3). EQU H.sub.1 (.omega.)=H.sub.R (.omega.)-.sub.j H.sub.I (.omega.) (3)
The impulse response of H.sub.1 (.omega.) is a Fourier conversion of the above described equation (3) and is expressed by the following equation (4). ##EQU2##
The impulse responses shown by h.sub.R (t) and -.sub.j h.sub.I (t) in the above described equation (4) are shown by the solid line and the dotted line, respectively, in FIG. 4. As seen from FIG. 4, these two impulse response characteristic curves indicate that the time interval between the respective peak points is 1/2f.sub.0 or 1/2.lambda..sub.0 in terms of the wave length and is uniform. These two impulse response characteristic curves further indicate that the respective peak points of two impulse response characteristic curves are positioned intermediate the peak points of the opponent curve. It is pointed out that the impulse response curve shown by the solid line is of the symmetrical component or the even component and the impulse response curve shown by the dotted line is of the non-symmetrical component or the odd component.
By composing the impulse response characteristic curves shown in FIG. 4, a new impulse response characteristic as shown in FIG. 5 is obtained, which represents h(t) shown in the above described equation (4), which is an impulse response of the desired frequency characteristic H.sub.1 (.omega.). Referring to FIG. 5, it is understood that electrode fingers should be disposed at the interval of 1/4f.sub.0 (1/4.lambda..sub.0), i.e. at the position corresponding to the respective peak points shown in FIG. 4, while the lengths of the electrode fingers are selected such that the overlapping lengths of the adjacent electrode fingers that are overlapped with each other are proportional to the amplitudes of the respective points shown in FIG. 5. The interdigital electrode thus obtained is shown in FIG. 6. As seen from FIG. 6, the electrode pitch is selected to be a constant value of 1/4.lambda..sub.0 and the overlapping lengths of the adjacent electrode fingers that are overlapped with each other are selected to be proportional to the amplitude shown in FIG. 5. It has been observed that the frequency characteristic of the filter thus obtained by employing the interdigital electrode shown in FIG. 6 achieves the desired characteristic, i.e. H.sub.1 (.omega.) shown in FIG. 2. It is pointed out that in the FIG. 6 example the width of the respective electrode fingers 11 to 15, . . . and 21 to 26, . . . of a pair of comb shaped electrodes 10 and 20 is selected to be the same as the electrode spacing and both the width and the electrode spacing have been selected to be 1/8.lambda..sub.0 for the reason that such is most preferred from the standpoint of designing. These electrode fingers 11 to 15 are coupled to a common electrode 10a at one end of each of them, whereby these electrode fingers are maintained in the same potential as that of the electrode 10a, while the electrode fingers 21 to 26 are coupled to a common electrode 20a at one end of each of them, whereby these electrode fingers are maintained in the same potential as that of the electrode 20a.
The mirror method is described in detail, for example, in ELECTRONICS LETTERS 28th Nov. 1974, Vol. 10 No. 24 "SYNTHESIS OF ACOUSTIC-SURFACE-WAVE-FILTERS USING DOUBLE ELECTRODES" and U.S. Pat. No. 3,968,461, issued July 6, 1976 to Richard Frank Mitchel et al. Briefly described, according to the mirror method, assuming that the central frequency of a desired frequency characteristic to be f.sub.0, an image of the central frequency of 3f.sub.0 which is line symmetrical with respect to the frequency 2f.sub.0 is considered and an impulse response thus achieved is similar to that in the case of the above described odd/even function method. Accordingly, the electrode pattern of the interdigital electrode is similar to that shown in FIG. 6.
However, the interdigital electrode achieved by such conventional odd/even function method or mirror method still involves a further problem to be described in the following. Described with reference to FIG. 6, with the interdigital electrode shown in FIG. 6, the symmetrical component is excited or received by the electrode fingers 11 and 21, 22 and 12, 13 and 23, 24 and 14, 15 and 25, and so on, while the non-symmetrical component is excited or received by the electrode fingers 11 and 22, 22 and 13, 13 and 24, 24 and 15, 15 and 26, and so on. In other words, the electrode pitch of the electrode fingers for exciting or receiving the non-symmetrical component is 1/2.lambda..sub.0, while the electrode pitch of the electrode fingers for exciting or receiving the symmetrical component is 1/4.lambda..sub.0. Accordingly, the length of the portion of the electrode fingers, say the electrode finger 22, for exciting or receiving the non-symmetrical component interacting with the electrode finger 11 must be selected approximately two times that in the case of excitation with the original electrode pitch of 1/4.lambda..sub.0. Accordingly, according to the conventional electrode pattern as shown in FIG. 6, the total length of the electrode fingers in the length direction, i.e. in the direction orthogonal to the propagating direction of the acoustic surface wave, is still long. Another problem is that since only the non-symmetrical component is excited or received with the electrode pitch of 1/2.lambda..sub.0 a difference in the propagation velocity of the acoustic surface wave is caused between the symmetrical component and the non-symmetrical component and thus a phase difference is caused at the output between the symmetrical component and the non-symmetrical component, with the result of an unfavorable effect on the frequency characteristic. Thus, these problems become obstacles in embodying the above described odd/even function method and mirror method. Considering these methods from the standpoint of fabrication of a filter, the width of the electrode fingers is 1/8.lambda..sub.0 in the case of the electrode patterns shown in FIG. 6, and, therefore, assuming that the central frequency f.sub.0 is 56.5 MHz, for example, then the actual width of the electrode fingers is about 5 .mu.m, which is too thin to fabricate the electrode fingers in accordance with the current technology. Therefore, a further problem is encountered that the electrode fingers are liable to be broken in an etching process and in further processes in fabricating the filter.